用途
观察者模式是一种
行为型模式。
结构
图-观察者模式结构图
Subject : 主题类，保存所有订阅此主题的观察者，观察者的
数量是任意的。定义
添加观察者
(
Attach) 和
删除观察者 (Detach) 的接口。
abstract
class Subject {
protected String name;
protected String state;
protected List<Observer> observers = new ArrayList<Observer>();
public abstract String getState();
public abstract void setState(String state);
public abstract void Notify();
public Subject(String name) {
this.name = name;
}
public void Attach(Observer observer) {
observers.add(observer);
}
public void Detach(Observer observer) {
observers.remove(observer);
}
}
protected String name;
protected String state;
protected List<Observer> observers = new ArrayList<Observer>();
public abstract String getState();
public abstract void setState(String state);
public abstract void Notify();
public Subject(String name) {
this.name = name;
}
public void Attach(Observer observer) {
observers.add(observer);
}
public void Detach(Observer observer) {
observers.remove(observer);
}
}
Observer : 观察者类，定义更新接口 (Update)，当收到 Subject 的通知时，Observer 需要同步更新信息。
abstract
class Observer {
protected String name;
protected Subject subject;
public Observer(String name, Subject subject) {
this.name = name;
this.subject = subject;
}
public abstract void Update();
}
protected String name;
protected Subject subject;
public Observer(String name, Subject subject) {
this.name = name;
this.subject = subject;
}
public abstract void Update();
}
ConcreteSubject : 具体主题类，存储对于这个主题感兴趣的所有观察者。当内部状态发生变化时，应通知所有登记的观察者 (Notify)。
class ConcreteSubject
extends Subject {
public ConcreteSubject(String name) {
super(name);
}
@Override
public String getState() {
return state;
}
@Override
public void setState(String state) {
this.state = state;
}
@Override
public void Notify() {
System.out.println("======= " + this.name + "主题发布新消息 =======");
for (Observer observer : observers) {
observer.Update();
}
}
}
public ConcreteSubject(String name) {
super(name);
}
@Override
public String getState() {
return state;
}
@Override
public void setState(String state) {
this.state = state;
}
@Override
public void Notify() {
System.out.println("======= " + this.name + "主题发布新消息 =======");
for (Observer observer : observers) {
observer.Update();
}
}
}
ConcreteObserver : 具体观察者类，实现 Observer 的更新接口 (Update)，以便和 Subject 同步状态信息。
class ConcreteObserver
extends Observer {
private String state;
public ConcreteObserver(String name, Subject subject) {
super(name, subject);
}
@Override
public void Update() {
state = subject.getState();
System.out.println( this.name + "收到当前状态：" + state);
}
}
private String state;
public ConcreteObserver(String name, Subject subject) {
super(name, subject);
}
@Override
public void Update() {
state = subject.getState();
System.out.println( this.name + "收到当前状态：" + state);
}
}
测试代码
public
class ObserverPattern {
public static void main(String[] args) {
ConcreteSubject subject = new ConcreteSubject("天气");
ConcreteObserver observer1 = new ConcreteObserver("张三", subject);
ConcreteObserver observer2 = new ConcreteObserver("李四", subject);
ConcreteObserver observer3 = new ConcreteObserver("王五", subject);
subject.Attach(observer1);
subject.Attach(observer2);
subject.Attach(observer3);
subject.setState("今天下雨");
subject.Notify();
subject.Detach(observer2);
subject.setState("明天天晴");
subject.Notify();
}
}
public static void main(String[] args) {
ConcreteSubject subject = new ConcreteSubject("天气");
ConcreteObserver observer1 = new ConcreteObserver("张三", subject);
ConcreteObserver observer2 = new ConcreteObserver("李四", subject);
ConcreteObserver observer3 = new ConcreteObserver("王五", subject);
subject.Attach(observer1);
subject.Attach(observer2);
subject.Attach(observer3);
subject.setState("今天下雨");
subject.Notify();
subject.Detach(observer2);
subject.setState("明天天晴");
subject.Notify();
}
}
运行结果
======= 天气主题发布新消息 =======
张三收到当前状态：今天下雨
李四收到当前状态：今天下雨
王五收到当前状态：今天下雨
======= 天气主题发布新消息 =======
张三收到当前状态：明天天晴
王五收到当前状态：明天天晴
张三收到当前状态：今天下雨
李四收到当前状态：今天下雨
王五收到当前状态：今天下雨
======= 天气主题发布新消息 =======
张三收到当前状态：明天天晴
王五收到当前状态：明天天晴
